1 Introduction

The intense development of anthropogenic activities in the last two decades has resulted in an increase in a load of pollution in the water bodies across the globe [1].

In lagoon systems, metal tends to accumulate in sediment in association with organic matter and it may be released into the water body due to changes in environmental conditions such as pH, dissolved oxygen, and temperature [2]. Mineral elements such as zinc, copper, manganese, iron, and calcium are classified as essential elements, and they play a lot of physiological functions in the body of living organisms. However, certain metals like lead, cadmium, chromium, arsenic, mercury, etc., do not play any essential role in the body and are highly toxic to all organisms even at low concentrations [3]. These metals are highly toxic, and because of that, they have been classified among priority metals by the European Union Commission (directives 2013/39/EU).

The marine environment has become contaminated due to a wide range of pollutants generated by various natural and anthropogenic activities, and this has prompted worldwide attention over decades [4, 5]. Processes like rapid industrial development, discharge from petrochemical industries, and waste discharged into the lagoon without proper treatment have contributed to the major sources of pollution. Also, the use of agrochemicals and fertilizers containing heavy metals from farmland settlements finds their way into many rivers [6]. During the transportation of these pollutants into larger water bodies like lagoons, there could be frequent changes due to dissolution, precipitation, and adsorption. Natural processes such as geological weathering of the earth’s crust and ocean tides could pose more stress on the lagoon water, thereby affecting the biodiversity of marine species as well as the ecosystem [7,8,9,10]. The contamination of the water body by heavy metals can be a potential threat to the environment, which can pose a risk to human health through the food chain because the water quality indices have an environmental and ecological impact on marine living organisms.

United Nations (UN) reports of the 2015 Millennium Development Goals (MDGs) showed that more than 40% of the global population was projected to be affected by water scarcity because of the accessibility to quality water resulting from the release of toxic pollutants into the lagoon system [11].

Human exposure to heavy metals has been linked to severe effects on human organs like kidney damage and various cancerous growths [6]. Hence, it is appropriate to have updates about the concentration of heavy metal present within and in areas of the harbour. Also, daily exposure to toxic chemicals could be established through major routes, which are, dermal, oral, and inhalation [12]. Health risk assessment is an important tool used for the evaluation of possible health effects caused by potentially toxic metals.

Studies have reported a lot of remediation processes for these diverse potentially toxic metal ions from wastewater by the use of innovative adsorbents, membrane separation, and photocatalytic-based treatment [13,14,15]. Different spectroscopic techniques like laser-induced breakdown spectroscopy (LIBS) and chromogenic analysis by the use of UV–visible spectroscopy have been employed in the quantitative evaluation of these diverse metal ions based on conjugated functionalized nanomaterials [16, 17]. Further research on the effective removal of toxic metal ions has also employed the use of ligand-based functionalized composite nanomaterials under acidic conditions [18,19,20].

Some of these potentially toxic metals which present as diverse metal ions in wastewater required sensitive and accurate analytical methods because of their selectivity as reported in the literature [14, 21, 22]

Lagos lagoon has been reported by several authors to be greatly polluted over the years, but the harbour [22], which is an important area of the lagoon system, has not been comprehensively addressed. It is against this background that necessitated this study.

Comprehensive monitoring of the Lagos harbour from November 2016 to July 2018 was conducted to assess the pollution status of the harbour in terms of contamination from anthropogenic pollution and its sources. The concentration of some priority metals was monitored in the lagoon water including some physico-chemical parameters. The carcinogenic and non-carcinogenic risks that could associate with these potentially toxic elements were also assessed based on exposure daily intake (EDI).

2 Experimental

2.1 Sampling and analytical procedure

All the glass that was used was washed with non-ionic liquid detergent and rinsed with potable water and later with purified water. The glassware was soaked in 0.1% nitric acid overnight and rinsed with ultra-purified water before being used. Water samples were collected from six different locations with the aid of a stainless-steel grab sampler into air-tightened amber bottles on the field during sample collections. In situ parameters were carried out on the water samples. The samples were transported to the laboratory on an ice chest. This was then followed by the addition of 0.1% nitric acid to the water samples for preservation. Table 1 contains the summary of the sampling points and the activities taking place, while Fig. 1 shows the map of the study area which translated the sampling points into pictorial form.

Table 1 Summary of the sampling points and their coordinates
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Fig. 1
figure 1

Map of study area

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2.2 Chemical and reagents

All reagents used were of analytical grades, and they were purchased from Merck in Germany, while Milli Q-water with conductivity less than 0.05 µs/cm was used for reagent preparation throughout the analysis.

2.3 Instrumentation

A Q-block automatic digester was used for the digestion of the water sample. An Agilent ICP-OES 4200 was used for the determination of heavy metals in the water samples. The Elmasonic P sonicator made from Germany was used in the course of sonication.

2.4 Quality assurance

To assure the accuracy of the data generated, a triplicate analysis of the samples was carried out. The blank samples were analysed at every ten determinations. A quality control standard was used to verify the accuracy of the machine.

2.5 Recovery study

Certified IAEA-SL-1 lake sediment was used for the recovery study of the method by weighing about 500 mg of the sediment and digested for total metal by aqua regia. The recovery values in the range of 85 to 110% were obtained for Cr, Cu, Ni, Pb, Al, Si, Fe, Cd, Sn with coefficients of variation less than 10%.

2.6 Statistical analysis of data

A series of statistical analyses have been applied by many researchers to analyse the results obtained from the analysis of metal based on the available software, such as SAS statistical package, Pearson’s correlation coefficient [3], and one-way analysis of variance (ANOVA) at P < 0.05. However, the results obtained during this study were analysed using statistical R-studio for the principal component analysis (PCAs) and one-way ANOVA [23] correlation of the physico-chemical analysis.

2.7 Sample preparation

Sample preparation was done in accordance with the method adopted by other researchers with modifications [2].

About 25 ml of the water samples was weighed into a 50 ml digester tube. 10 ml of concentrated nitric acid was added and placed inside an automatic digester for the digestion process.

The sample was heated and allowed to stay for about one hour inside the digester until the brown fume started to change to almost white. The digester machine was stopped, and the sample was allowed to cool and later made up to 25 ml mark with purified water. The sample was filtered through Whatman no 42 filter paper, and the clear filtrate was ready for analysis of heavy metals.

The standards of various concentrations for the metals of interest were prepared and run to establish linearity after which the samples were run. The amount of the metals present in the samples was calculated automatically by the machine.

3 Results and discussion

3.1 Lead

Lead has been classified by the United States Environmental Protection Agency [24] as potentially hazardous to aquatic organisms as well as human beings due to its neurological disorder and other toxic effects. In this study, the average concentration of lead varied from 2.55–5.85 mg/L in the dry season, while that of the wet season ranges from 2.40–4.46 mg/L as contained in Table 2, and the higher value was recorded in the dry season than in the wet season with the highest concentration detected in location E. This high concentration could be a result of effluent discharged containing metallic materials into the lagoon in addition to leaded fuel being used by boats and ferries on Lagos water [25, 26]. The concentration of lead detected in both seasons is above the WHO/USEPA limit (0.05 mg/L) for water. The higher value of lead detected during both seasons agreed with other researchers.

Table 2 Concentration of heavy metals (mg/L) in Lagos harbour
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3.2 Cadmium

Cadmium has been recognized as one of the metals on the priority list by EU directives due to its toxic effect on marine organisms and human beings even at low concentrations and its possible bioaccumulation by aquatic organisms [27, 28]. The concentration of cadmium detected ranges from 0.03 – 0.15 mg/L with location C having the highest concentration. The concentrations of cadmium measured during this study were lower than what was reported on the Awash River in Ethiopia [4]. However, the presence of cadmium in harbour water could be a result of atmospheric deposition of non-ferrous metals in addition to domestic waste discharged into the harbour from different point sources and also industrial discharge that contained used batteries and dye from the Apapa industrial region down to the lagoon [28].

3.3 Chromium

The diverse occurrences of chromium ions in the marine environment play a significant role in the toxicity of the metal [29]. The chromium concentration in the lagoon water varied from N.D–0.9 mg/L at an average of 0.05 mg/L. The amount of chromium detected in almost all the locations was below the WHO limit (0.1 mg/L) except in location C which was much higher during the wet season (0.9 mg/L) than the regulatory specification, and studies have reported that a high concentration of chromium in the body can interfere with the iron uptake by haemoglobin [30]. The higher concentration of Cr in this location could be attributed to effluent-containing electroplating material [31].

3.4 Copper

Copper is the third most used metal in the world because of its economic and industrial importance [32]. It remains an important microelement needed in the growth of plants and animals as well as haemoglobin fortification in human beings. The variation of copper during the seasons showed that a higher concentration of copper was measured during the dry season than in the rainy season at an average of 5.97 mg/L. The higher values were recorded across the locations.

3.5 Iron

Iron occurs mainly as a ferric or ferrous state which is an essential trace metal with significant metabolic activities in the body system [24] and requires a certain amount of water. Iron is an essential heavy metal that plays a vital role in the growth and metabolism of a living organism. The deficiency of iron in the body can lead to diseases, while the excess of it in the aquatic environment could be dangerous to marine organisms as well as human beings [33, 34]. The concentration of iron measured during this study on Lagos harbour was higher in the dry season than wet season which ranged between 6.53 and 13.6 mg/L with location D having the highest concentration. The higher value recorded in this location could be a result of industrial inputs from Amuwo odofin/Apapa that discharged their partially treated or untreated waste into the lagoon in addition to metallic waste from shipping activities. The average concentration of Fe measured across all the locations is much higher than the WHO limit (0.3 mg/L). A recent study on Lagos lagoon had shown the presence of a higher amount of iron during the dry season [35]

3.6 Nickel

This is one of the non-essential elements which can form complexes in an aquatic environment with organic and inorganic materials which can absorb directly into clay particles or lagoon sediment [36]. The possibility of nickel co-precipitate with some anions and cations at pH greater than 5 as reported in the WHO peer review document can redistribute itself into the water phase [36,37,38]. Furthermore, the presence of nickel in Lagos lagoon water could be traceable to metallic scrap from the welding and battery industry situated around the harbour which discharges their waste into the lagoon.

3.7 Aluminium and silicon

The average concentration of aluminium ranges from 12.25–36.72 mg/L, while that of silicon ranges from 0.67–2.13 mg/L. The highest concentrations of these elements were recorded in location D. Location D is the extension of the harbour which serves as a reservoir for inputs from aluminium smelting industries located at Apapa. The possible discharge of partially treated waste into the harbour could be a contributing factor [39, 40] Also, there could be a possibility of natural deposition of these metals into the harbour which might have adhered to the lagoon sediment and eventually leached into the water body [41, 42].

3.8 Tin

The concentration of Sn ranges from 0.67–2.13 mg/L. Again, the highest concentration of Sn has been detected in location D. The detection of Sn across all the locations is an indication of the prevalence of the use of antifouling chemicals used in shipping activities [3, 43]. The metallic tin is not toxic, but when it is present in organic species, it becomes toxic, especially tri-organotin compounds, which are the basis of organotin compounds in aquatic environments [44].

3.9 Effect of pH and temperature on lagoon system

The pH of a medium is the level of acidity or alkalinity of the medium [45]. The pH of the lagoon is an important factor that affects the solubility and dispersibility of metals in the harbour.

The result of the physico-chemical analysis according to Fig. 2 showed that the pH of the Lagos harbour ranges from 6.2 – 6.60 during the wet season, while it was from 6.7–7.20 during the dry season. The highest pH value was recorded in location E. The pH of the harbour during this study is relatively neutral as this might not have a significant effect on marine organisms [46, 47]. The result of the pH values falls within the WHO standard for the level of acidity or alkalinity of the Lagoon water (6.5–8.5) [36].

Fig. 2
figure 2

The chart of pH variation across the locations

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The temperature is another important parameter which has a serious impact on the marine environment [48]. The mean temperature range during this study was from 24–30 °C. The results obtained according to Fig. 3 showed that the temperature recorded during the dry season is slightly higher than that obtained during the rainy season [49]. This could be a result of interaction between surface water and atmospheric temperature, which is usually high during the dry season, and this can lead to intense evaporation of the water [50]. Also, there could be possible mixing of surface water with underlying lagoon water, thereby leading to uniformity of the entire water column and consequently leading to higher temperatures during the dry season. Similar results have been reported in the works of the literature [50, 51]. These results were also in agreement with the range of temperature recorded by Oyeleke and his co-workers (27–30 °C) that worked on Lagos lagoon. However, the measured temperature was slightly higher than what was recorded by Adedayo and his co-researchers that worked on Lagos lagoon [52], but lower than the temperature recorded on Bonny island water in River State, Nigeria (32.2 °C ± 0.03) [16].

Fig. 3
figure 3

The chart of temperature variation across the locations

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3.10 Effect of electrical conductivity, total dissolved solid (TDS), and salinity

The conductivity of water is a measure of its ability to carry electric current as a result of various ions in the water medium [45]. The effect of the conductivity of a medium is reflected in the salinity and the level of total dissolved solids of the medium.

The range of results obtained for conductivity, TDS, and salinity during the dry season is (1.95–72.52 × 103 µs/cm), (1.02–38.08 × 103 mg/L), and (15.21–75.1 mg/L), respectively. Location D has the highest value, while the results obtained during the wet season are (1.86–42.52 × 103 µs/cm), (0.93–24.62 × 103 mg/L), and (10.50–22.80 mg/L), respectively. The results obtained during the dry season are higher than that observed during the wet season as shown in Fig. 4; this showed that there is a significant difference at (P < 0.05) in seasonal variation and this could be a result of the increase in the levels of cations and anions due to possible evaporation in lagoon water which tends to increase the concentration of ions present in marine environments [50]. A similar result was observed by other researchers on marine water. Also, the higher values of electrical conductivities during the dry season could be a result of run-off of loaded potential toxic metals and other ions from chemical industries situated within the Apapa area of Lagos, which consequently led to higher total dissolved solids and salinity during the dry season as contained in Figs. 5 and 6, respectively [50].

Fig. 4
figure 4

The chart of conductivity variation across the locations

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Fig. 5
figure 5

The chart showing the variation in salinity

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Fig. 6
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The chart showing the variation in TDS across the locations

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3.10.1 Pearson’s correlation coefficient

The results of the Pearson’s correlation in Table 3 showed the interactions among the variables There is a strong positive correlation between pH and electrical conductivity EC, r = 0.96. Also, a significant correlation occurred between total dissolved solids (TDS and EC, r = 0.99) [41]. This strong correlation between TDS and EC follows the trend of regression analysis models for water quality parameters reported by other researchers [53]. The result of regression analysis for TDS and salinity showed a strong correlation between the two variables, r = 0.90, which is in agreement with what has been reported in the literature [54, 55]. However, a weak correlation between EC and TDS on river Benue in Makurdi, Nigeria, was reported by researchers (TDS, EC, r = 0.34) [56]. Furthermore, weak correlation occurred between the pH and DO, r = 0.48, while a negative correlation exists between DO and EC, r = −0.51. A similar pattern of correlations was observed by other researchers that worked on variations of physico-chemical parameters in the Lagoon [4].

Table 3 Correlation coefficient of physico-chemical parameters measured in Lagos harbour
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3.10.2 Principal component analysis (PCA)

The statistical R-studio was used to perform PCA of the heavy metal of the data obtained during the study. The principal component analysis was utilized to identify the origin of heavy metals in water and was shown to be an efficient tool to define anthropogenic sources of heavy metals. The result of PCA for heavy metal contents in water is shown in Fig. 7. The first two component factors were extracted from the plot. As a consequence, heavy metals can be grouped into 2 components in which component 1 explained approximately 54.9% of all the data variation. Potential toxic metals such as Pb, Cd, Fe, Ni, and Cu with high coefficient values are highly correlated, and this showed that these metals would most likely originate from industrial and domestic discharges that contained these heavy metals [39]. The second component (PCA 2) accounts for about 20% variance and that Si, Al, Cr, Sn variation in the heavy metal concentration in water in location D, Si, Al and Cr are highly correlated and concentrated in that location, which suggests that these metals could originate from natural sources, while the Sn contaminant could be from ship repairing and other anthropogenic sources [57].

Fig. 7
figure 7

Principal component analysis for the metals across the locations

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3.10.3 Cluster analysis (CA) on potentially toxic elements in Lagos lagoon water

Cluster analysis was performed based on squared Euclidean distance and Ward’s linkage method in line with some researchers [2, 58]. From Fig. 8, two cluster plots with different geochemical characteristics can be identified. The first plot tetragonally grouped Cu, Pb, Cd, Si, Ni, Cr, and Sn. This pattern of grouping indicated that Cu, Cr, and Ni could originate from the same source point, which is likely to be from industrial inputs from neighbouring companies around the harbour [59]. Also, Pb and Sn could be as a result of boating activities, which is a usual practice around the Lagos harbour that uses leaded gasoline as a fuel source which might get absorbed onto the lagoon sediment and eventually remobilized into the water compartment during dredging and other physico-chemical processes [42]. The presence of Si in Lagoon water could be attributed to the geogenic nature of the element [60]. The second cluster that grouped Fe and Al showed that apart from anthropogenic activities that involved painting and welding within the harbour that generate these two metals [2, 25, 28], these metals could also be attributed to their geochemical natural abundance. [61,62,63]. The dendrogram in Fig. 9 also showed the hierarchical interaction among these metals.

Fig. 8
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Cluster analysis of heavy metals in Lagos lagoon water

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Fig. 9
figure 9

Dendrogram for heavy metals in Lagos lagoon water

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3.10.4 Risk assessment

Human risk assessment comprises the determination of the type and extent of adverse effects on human health of prolonged exposure to toxic contaminants [64]. The health risk of potentially toxic elements is usually based on the estimation of potentially carcinogenic and non-carcinogenic analysis. The carcinogenic and non-carcinogenic health risk assessment based on oral and dermal exposure routes was calculated according to equations described by USEPA risk assessment methods [12, 65].

3.10.5 Non-carcinogenic risk equations

Oral route:

$$ HQ_{D} \; = \;\left[ {Kp \times CW \times t_{{{\text{event}}}} ~ \times EV \times ED \times EF \times SA} \right] \div BW \times ATnc \times RfDo \times ABS_{{Gi}} $$
(1)

Dermal route:

$$ HQ_{D} \; = \;\left[ {Kp \times CW \times t_{{{\text{event}}}} ~ \times EV \times ED \times EF \times SA} \right] \div BW \times ATnc \times RfDo \times ABS_{{Gi}} $$
(2)

3.10.6 Carcinogenic risk equations

Oral route:

$$ {\text{Risk}}_{o} \; = \;\left[ {CW \times IR~ \times EF \times ED~ \times SFo} \right] \div \left[ {BW \times ATc} \right] $$
(3)

Dermal route:

$$ {\text{Risk}}\;D{\text{ }} = \;\left[ {Kp \times CW \times t_{{{\text{event}}}} ~ \times EV \times ED \times EF \times SA~ \times SFo} \right] \div \left[ {BW \times ATc \times ABS_{{Gi}} } \right] $$
(4)

where IR, EF, ED, EV, BW, SA, AT, and t event are defined in Table 4 [76, 77].

Table 4 USEPA default parameters used for oral and dermal exposure assessment
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HQo and HQD are hazard quotients (non-carcinogenic risk values) for oral and dermal ingestion, respectively, while Risko and RiskD represent the carcinogenic risk values for oral and dermal routes, respectively. CW is the mean concentration of potentially toxic metals measured in Lagos harbour water. ABSGiis the gastrointestinal absorption fraction, SFo is the oral slope factor, RFDo is the oral reference dose, and Kp is the permeability coefficient. Table 4 indicates the default parameters for the exposure route assessment of toxic contaminants, while Tables 5 and 6 show the default parameters for cancer toxicity data and non-cancer toxicity data of chemicals, respectively.

Table 5 Cancer toxicity data
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Table 6 Summary of non-cancer toxicity data
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3.10.7 Non-carcinogenic risk

From the results in Table 7 of four potential toxic metals that were risk assessed, the non-carcinogenic risk values (hazard quotient) measured for Cd, Cr, and Pb were greater than 1 based on the estimated daily intake via oral exposure, this is an indication of possible health risk in both the children and adults that might engage in activities within and along the Lagos harbour [67, 68], and the literature has reported direct severe impact of these three potential toxic metals in vivo body system that could result in cancerous growth over some periods [66]. However, the non-carcinogenic risk assessment carried out by this study showed that dermal exposure to Cd and Cr in both adults and children might not pose a significant health risk because their HQ values were much less than 1 [69].

Table 7 Calculated non-carcinogenic and carcinogenic risk value
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3.10.8 Carcinogenic risk

The results of the carcinogenic assessment are summarized in Table 7. The risk index is in the order Ni > Pb > Cr > Cd. The risk values through the oral medium from this study for Ni, Pb, and Cr were greater than the daily exposure limit (1 × 10−6 mg/kg/day) as recommended by USEPA 2010. The higher values for these metals are indications of an incremental potential cancer in both children and adults as reported by other researchers [66]. The literature has reported that Pb, Cr, and Ni could interact with selenium, which is an essential element in the body’s metabolism. This interaction can increase in vivo cancer growth which can consequently put both adults and children at risk. [66, 70]. The results also revealed that dermal exposure to Cr could cause an incremental cancer risk in both adults and children as can be seen by their cancer risk values of 3.17 × 10–5 and 1.08 × 10–4 mg/kg/day [64] Also, continuous exposure to lead (Pb) could have serious health implications in the blood of children and adults as reported in works of the literature [64].

Overall, the total non-carcinogenic risk values measured for Cr and Cd in this study were higher in children than adults; this is an indication that there could be more adverse and higher hazardous risk effects of these metals on children than in adults [66, 71]. Hence, children are more vulnerable to these toxic metals, most especially those children involved in fishing and other boating activities along Lagos harbour. The results obtained were in agreement with the study conducted by other researchers about the risk effects of Cd and Cr on adults and children [72]. The toxicity effects of the combined metals (Cr and Cd) on marine organisms have been reported to be severe in the literature as a result of the toxicological pattern of these metals on human being [29, 73].

3.10.9 Discussion of Table 8

Table 8 Comparison of the results (mg/L) of present study with those reported in the literature works
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The results of PTEs like Al, Fe, Ni, and Pb measured in Hooghly River by Ghosh and his co-workers according to Table 8 exceeded the permissible WHO limits. These higher concentrations, according to the authors, were attributed to rapid and unplanned urbanization coupled with industries in the neighbourhood of river Hooghly [76]. This trend reflected what Sticker and Lyon carried on a similar river. On comparing these results with the present study, it showed that the concentration of Pb is much greater than that of the present study.

Dharfer and his group of researchers measured six potential toxic metals in the mangrove lagoons of the red sea coastal cities, Saudi Arabia. The average concentrations of PTEs detected in this river were much greater than in the present study as contained in Table 8. The higher values for these measured toxic metals in this river were attributed to an increase in sewage water discharged into the lagoon [31]. Also, the river Jeddah, which is one of the biggest red sea coastal cities in Saudi Arabia, has historical inputs of over two decades of refuse from metallic waste which perhaps might have accumulated down the sediment and later remobilized into the water compartment.

Kortei and his co-researchers detected cadmium and lead in the Pra basin in Ghana as shown in Table 8. The results of these two PTEs measured were below the WHO limits. This indicated that the presence of these metals at this concentration might not pose a significant threat to the quality of the water. However, when the risk assessment of these metals was carried out based on the daily intake exposure (EDI), the total hazard quotient for Cd was greater than 1 in both adults and children and this means that there could be likely adverse effects of prolonged exposure to these toxic metals [72].

Okoro et al. 2016 measured PTEs in Cape Town harbour. The concentrations of those elements measured according to Table 8 were higher than the WHO limits. These higher values, according to the authors, were a result of rapid urbanization in a developing country like South Africa, which has led to the generation of a lot of storms and discharges into the harbour. Cape Town harbour is also one of the busiest harbours in terms of shipping activities which could be responsible for the higher value of the PTEs in the harbour [25].

Don Pedro and his group of researchers studied the distribution and occurrence of some potentially toxic elements (PTEs) in the Lagos lagoon. Significant amounts of these PTEs were measured according to Table 8. The high concentrations of these metals in the Lagos lagoon are attributed to the metal-laden industrial and domestic effluents that enter the lagoon via drainage channels [31], which have led to high pollution rates. Also, the non-degradability of these metals has enhanced the slow rate of desorption of these metals into the lagoon water compartment. Therefore, there could be a high risk associated with the prevalence of these PTEs in the Lagos lagoon. The results obtained by Don Pedro were much higher than in the present study. Furthermore, Adebola and her group of researchers carried out a comparative study about a decade after Don Pedro worked on Lagos lagoon, according to Table 8. The result of their analysis showed that most of the PTEs in Lagos lagoon have been reduced significantly (P < 0.05) compared to what was reported by Don Pedro. These improvements were attributed to enforcement by regulatory bodies like the Federal Environmental Protection Agency (FEPA) and Lagos State Environmental Protection Agency (LASEPA) who monitor and regulate the discharge of industrial effluents [75]

4 Conclusion and recommendation

This research work focused on the assessment and distribution of potentially toxic elements (PTEs) in the Lagos lagoon. This study has significant information about the quality of water collected from Lagos harbour. The major findings of this research showed that some PTEs like lead, cadmium, copper, iron, and nickel were above the WHO limits, and this could pose a great threat to the ecosystem. The discharge of industrial and domestic waste in addition to the use of historical inputs from antifouling activities has contributed greatly to the high concentration of potentially toxic elements in the harbour. Also, the correlation coefficient of the physico-chemical parameters showed the interdependence of these variables. The summary of the comparative analysis of this study with the few studies across the world in Table 8 reflected that PTEs contamination of lagoon waters is a global problem because most of the reported results were much higher than the regulatory limit.

Furthermore, the finding from the risk assessment showed that continuous exposure to some metals (Ni, Cd and Pb) by adults and children through various activities like boating, diving, and fishing could lead to an incremental potential cancer, damage to other organs like brain and lung, because the calculated risk indices of these metals were greater than (1 × 10–6 mg/kg/day) as recommended by USEPA 2010. The findings have reflected the polluted nature of Lagos harbour.

Limitations, the major challenges with this work, were that during the rainy season, the harbour used to be deeper and sampling with the use of grab sampler while on a speed boat was not convenient, in that case, the diver was employed which resulted in extra cost.

In conclusion, this research work has set a template and baseline for the monitoring of other harbours in Nigeria. This is very important because a lot of shipping and boating activities are still going on in Nigeria harbour which could generate toxic elements. This research work has provided evidence of the presence of organotin compounds and their associated metabolites in the harbour which predominantly resulted from anthropogenic activities. In addition, this research has provided novel quantitative data on nine potentially toxic elements in Lagos harbour which have not been previously studied by any authors on the Nigeria harbour system. Also, the risk assessment study has provided evidence of environmental contamination which could lead to marine extinction if not properly managed. This study will help in facilitating effective approaches for the management of lagoons in Nigeria to meet up with the environmental sustainability of the Millennium Development Goals (MDGs).

It is therefore recommended that there should be public enlightenment among the stakeholders. Also, the combination of close monitoring and improvement on legislative regulations within and around the Lagos lagoon is recommended. Further research should be tailored to the areas of eco-friendly remediation of the harbour by the use of highly selective and innovative composite nanomaterials, photocatalytic-based treatment for effective adsorption of diverse toxic metal ions from the Lagoon system to maintain environmental sustainability as well as safeguard the health of the populace.